1. Field of the Invention
This invention relates to molds for the manufacture of glass products, for example bottles.
2. Description of the Related Art
The manufacture of glass products is preferably carried out in two main stages: In the initial stage, the glass gob, preferably heated to a temperature of approximately 1100.degree. C. is placed in contact with a mold. The mold preferably comprises a rough-forming mold that has connected at its lower portion a ring mold. The portion of the gob in contact with the ring mold is preferably cooled to a point where the glass has almost solidified into its final shape as the neck of the glass product. The remaining portion of the gob is preferably kept at a high enough temperature to permit the further shaping and sculpting of this part of the glass product.
The different temperatures of the glass gob in the rough-forming mold and the ring mold are preferably produced by external cooling means, including the blowing of ventilated air on the sides of the ring mold. Different temperatures in the two molds are also preferably produced by the use of different materials for the ring mold and for the rough-forming mold. In this process, the material used to make the ring mold preferably possesses a higher thermal conductivity (or diffusivity), than the material used to make the rough-forming mold. As a result, heat is transferred away from the gob more rapidly in the ring mold than in the rough-forming mold and the temperature of the portion of the gob in the ring mold is lower than the gob temperature in the rough-forming mold.
After the glass gob is shaped in the rough-forming mold, it is preferably placed in a finishing mold. Finishing molds have to absorb a greater quantity of heat than the rough-forming molds, because the glass products in these molds generally drop to temperatures below the solidification temperature of glass.
Conventional methods for making the molds described above preferably include casting the molds. Unfortunately, the casting methods used produce molds that have coarse surfaces and that frequently crack due to thermal fatigue.
Bronze is an alloy that is preferably used in the molds and is very prone to corrosion and oxidation. In contrast, marine bronze produces molds with a very fine surface structure, good resistance to corrosion, and good resistance to cracking due to thermal fatigue. Marine bronze is preferably made from approximately 9-10% wt. Al, 4-5% wt. Fe, 4-5% wt. Ni, and the balance comprising Cu and customary impurities.
The manufacture of glass products as described above requires high temperatures, preferably 900-1300.degree. C., more preferably 1000-1200.degree. C., even more preferably 1050-1150.degree. C., and rapidly alternating thermal cycles, preferably 10 cycles/minute. Under these conditions, even molds made from marine bronze undergo a surface oxidation that forms pits and blemishes on the surface of the mold.
The erosion of the mold surface is further accelerated in a preferred glass manufacturing method that involves coating the surface of the mold with grease prior to contact with the glass gob: In this method, the grease forms graphite deposits on the oxidized portions of the mold at high temperatures. The graphite deposits accelerate the wearing away of the surface of the mold.